Digital microfluidic systems based on electrowetting are attractive because they enable fluid actuation by electrical signals alone, thereby reducing the complexity of the chip as well as the overall system. Such electrically driven microfluidic devices, particularly using the EWOD chip configurations, have shown promise as generic platforms because they can be reprogrammed on-the-fly using software to accommodate various biochemical protocols. Further, because EWOD chips have low power requirements, it is conceivable to build a handheld (e.g., cellphone-like) system that runs on batteries.
Most EWOD-based digital microfluidic devices have one of the following generic architectures: parallel-plate or open-planar. Of these, the former is often preferred, despite the additional steps in fabrication, for its utility and reliability in applications. For example, it is far easier to generate droplets and split them apart when a liquid is squeezed between two parallel plates. Also, controlling the device gap (between the plates) is a reliable way to scale the working fluid. In comparison, on an open-planar device, droplet dimensions are determined by the contact angle, which is nearly impossible to control or predict with accuracy. Third, if the gap is much smaller than the capillary length (e.g., 2-3 mm for water), large droplets are insensitive to inertial forces. The same cannot be said for open-planar devices unless the droplet volume is small, i.e., the diameter is much smaller than the capillary length.
Traditional parallel-plate EWOD device manufacturing begins with thin film deposition and patterning of the two plates, typically a bottom substrate patterned with EWOD electrodes and a top plate with a blank conductive layer, and ends with assembly. FIG. 1 illustrates an EWOD device 2 according to the prior art. The EWOD device 2 includes a transparent top plate 4 and a bottom EWOD substrate 6 (e.g., plate) that includes surface electrodes 8. A spacer 10 is used to separate the top plate 4 from the lower plate 6. Overall, the plates 4, 6 are fabricated in the clean room, for example, using one step each of metallization, photolithography, and dielectric deposition. The assembly process, which can be performed using a variety of methods, involves alignment of the two plates 4, 6 (which is not critical for most applications because the ground plate is unpatterned), fabricating and positioning spacers 10, and bonding of the plates 4, 6. The last two steps determine the gap between the plates 4, 6 and therefore the thickness of the droplets 12. It is common to align and affix cutouts of an adhesive spacer, e.g., double-sided tape, to the plates for spacing and bonding. While this practice is acceptable for fabricating parallel-plate EWOD devices with relatively large gaps (i.e., >50 μm), when smaller gaps are desired, a thick photoresist (e.g., SU-8) can be coated and lithographically defined as a spacer, and additional provisions, e.g., external alignment and clamping mechanisms, are employed to hold the plates together.
While EWOD devices can be formed using the two plate construction described above, there is a need for EWOD devices that use smaller and smaller droplets down to the micrometer scale (e.g., sub-nanoliter volumes). Moreover, the alignment and assembly steps required to form the two plate construction adds cost and complexity. There is a need for an improved EWOD device and method of making the same that is able to provide form small and uniform gaps that enable picoliter-sized droplets to be manipulated in the EWOD device.